This invention relates to improvements in walkie low-lift trucks. Such materials handling trucks are normally self-propelled and are used to pick up, transport and deposit loads in certain warehouse and loading dock applications where the loads need not be lifted to a great height. In such applications where high lift is not a requirement, the greater maneuverability and lower cost of such low-lift trucks provide a significant advantage over conventional high lift, rider-operated lift trucks.
Structurally, a walkie low-lift truck conventionally comprises a main frame, with which are associated a power supply, motor, controls and a pivotally mounted driver-steer wheel, and a load-carrying structure in the form of a parallel-arm fork or generally laterally disposed platform extending from the front of the main frame and movable between a lowered, loading and discharge position and a raised, load-transporting position. In one general type of low-lift truck, the upper surface of the load-carrying structure is capable of being lowered only to a limited extent such that the structure can be inserted within a space existing between the floor and the bottom of the load, such space being ordinarily provided by a wooden pallet or other similar means for supporting the bottom of the load a distance above the floor. Examples of this type of low-lift truck, usable only with loads supported a spaced distance above the floor, are shown in Loeb U.S. Pat. No. 734,076, Kennedy U.S. Pat. No. 2,299,150, Becker U.S. Pat. No. 3,072,418, Trusock et al U.S. Pat. No. 3,183,989, Nichols U.S. Pat. No. 3,246,713, Goodacre U.S. Pat. No. 3,352,569, Leonard et al U.S. Pat. No. 3,441,287, and Harrison et al U.S. Pat. No. 3,625,534.
In a second general type of low-lift truck, more versatile than the type just described in that it is capable of handling nonpalletized, as well as palletized, loads, the load-carrying structure, particularly in the form of a tiltable platform called a "platen", is arranged to tilt forward while being lowered until a forward edge touches the floor. Thus in the lowered position the platen forms an inclined plane to facilitate loading and discharging of loads supported merely on paper sheets known as "slip sheets". A push-pull mechanism is commonly used with this type of low-lift truck to mechanically pull a slip sheet, and thereby its load, up the inclined plane of the platen for loading, or push the load down for unloading. It will be appreciated that, for the loading of slip sheet-supported loads, it is imperative that the forward edge of the upper surface of the platen be capable of being tilted into close proximity with the floor at the juncture between the slip sheet and the floor to enable the slip sheet to be pulled onto the platen.
In present low-lift trucks having such tiltable platens, the lowering and lifting of the forward edge of the platen is accomplished in different ways. One way is exemplified by the low-lift truck shown in Rigsby U.S. Pat. No. 3,495,730 where load-supporting rollers are reciprocated forwardly and rearwardly through a substantial distance to raise and lower respectively the forward edge of the tiltable load-carrying structure. This type of mechanism has a major drawback in that it cannot provide a sufficient degree of articulation between the tiltable structure and the main frame to satisfy many load-handling conditions. In particular, the degree of elevation provided by extended the rollers forwardly is limited substantially to the diameter of the rollers, which in turn is limited by the available space for storage of the rollers in their retracted position. This limited degree of elevation makes it difficult or impossible for this type of low-lift truck to be used interchangeably for the handling of palletized and nonpalletized loads since the limited degree of elevation provided may be insufficient to elevate a palletized load or, alternatively, insufficient to provide necessary clearance between the pallet and the floor to transport the palletized load over floor surfaces having normal surface variations, changes of slope and the like. Moreover, with respect to the handling of slip sheet-supported loads, it is often necessary to pick up the load while the main frame of the truck is supported on a surface the plane of which is angled with respect to the surface upon which the load is supported. For example, in typical loading dock operations it is common for the bed of a freight-carrying truck to be at a somewhat different elevation than the surface of the dock. In such cases a ramp, sloping either upward or downward from the dock, communicates between the truck bed and dock to enable a lift truck to travel between the two. In attempting to pick up the first, or rearward, loads from a fully loaded truck, the main frame of the low-lift truck is thus normally positioned on the ramp at an angle with the truck bed. If the ramp slopes upwardly from the dock, it will be necessary that the forward edge of the platen be depressed to a level below the plane of the ramp surface so as to contact the truck bed at its interface with the slip sheet. Conversely, if the ramp slopes downwardly from the dock, the forward edge of the platen may need to be elevated substantially just to be able to engage the load initially, and then further elevated in order to raise the load to a transporting position. For the same low-lift truck to be able to accomplish all of the foregoing different maneuvers a higher degree of articulation between the platen and the main frame of the truck, and a concomitant greater range of elevation of the forward edge of the platen, than can be provided by the mere forward and rearward reciprocation of rollers as in the Rigsby device must be provided.
A type of structure which is capable of providing the necessary degree of articulation and range of elevation of a tiltable platen is one wherein the elevation is accomplished by powered rotation of load-supporting struts depending pivotally from a load-supporting frame beneath the platen, on the depending ends of which are mounted load-supporting, floor-engaging wheels or rollers. Strut structures are presently used for elevating the tiltable platens of low-lift trucks, as evidenced for example by Fernstrom et al U.S. Pat. No. 3,392,858 and Rocco U.S. Pat. No. 4,065,012. In the raised position of these platens, the struts extend generally vertically downward so as to maximize the distance between floor and platen. Powered means fold the struts to lower the load-supporting frame and forward edge of the tiltable platen by pivoting the struts in a forward direction, thereby moving the wheels forwardly away from the main frame of the truck. Conversely, the struts are pivoted in the rearward direction to resume a vertical position in order to raise the forward edge of the platen to its load-transporting position, thereby moving the load-supporting wheels rearwardly toward the main frame of the truck. However, in moving rearwardly during the transition from the lowered position to the raised, load-transporting position of the platen, the load-carrying wheels cause a shortening of the wheel base and thereby reductions in load-carrying capacity, stability, tractive effort and steerability of the truck. These reductions are peculiar to the tiltable platen type of low-lift truck because the tiltable platen requires that the location of the load-supporting struts and wheels be a substantial distance rearwardly of the forward edge of the platen to enable the forward edge to be inserted beneath a slip sheet. Such rearward location of the struts and wheels in turn insures that the center of gravity of most loads on the platen will be forward of the wheels, counterbalanced by the weight of the main frame. Thus, as the load-supporting struts and wheels move rearwardly to raise the platen in present tiltable platen type trucks, the load's forward tipping moment about the wheels increases, tending to overcome the counterweight provided by the main frame. This condition can cause the truck to become unstable and unsteerable and to lose tractive effort as a load is raised, despite adequate initial stability, steerability and traction. Thus while a load is being lifted the truck may suddenly tilt forward, pivoting about the axles of the load-carrying wheels and raising the drive wheel from the ground. Even if the drive wheel is not lifted from the ground, the rearward movement of the load-supporting wheels as the load is raised inevitably lessens the portion of the total load carried by the drive wheel, reducing the static stability of the truck in the fore and aft direction and reducing the steering control and tractive effort afforded by the drive wheel.
Conversely, in folding the struts forward to lower the forward edge of the platen, the present tiltable platen type trucks limit the maximum degree of depression of the platen which can be obtained. Such limitation on the depressability of the platen can be particularly unsatisfactory in loading dock operations where the bed of a freight-carrying truck is higher than the surface of the dock, as described above.
Accordingly, what is needed is a low-lift truck, of the type having a tiltable platen, which: (a) provides an improved degree of articulation between the platen and main frame, particularly with respect to depression of the platen; (b) provides a sufficient degree of platen elevation to handle palletized as well as nonpalletized loads; and (c) increases; rather than decreases, the load-carrying capacity, forward stability, steerability and tractive effort of the truck as the load is elevated by the platen.